This application for a K05 Senior Scientist Award requests an additional 5 years of support from NIDA for Nancy Zahniser, Ph.D., Professor of Pharmacology and Neuroscience, U Colorado School of Medicine. The U Colorado is distinguished by its strong drug/alcohol addiction research, funding record, training programs and collaborative atmosphere. Dr. Zahniser's support by a NIDA K05 Award for the past 4 years has been crucial in allowing her to focus on and expand the scope of her drug addiction research. Her research seeks to understand how the differential initial behavioral responsiveness of individual rats to cocaine predicts their vulnerability to addiction, how the dopamine transporter (DAT) contributes to these phenotypes and how functional DAT expression is regulated. Recently, she began a new collaboration with Dr. Richard Allen that has enhanced her lab's behavioral testing repertoire to include conditioned place preference (CPP) and, very soon, self-administration. Overall, her work has shown that the initial locomotor response to cocaine can predict the extent of cocaine-induced locomotor sensitization and CPP, suggesting that factors like DAT that contribute to the different phenotypes may help to explain individual differences in reward. Such findings have sparked her interest in understanding how DAT is regulated and her productive collaboration with Dr. Alexander Sorkin. DAT regulation occurs largely via altered trafficking and cell surface expression. Their work has resulted in novel insights about the cellular mechanisms utilized during endocytic DAT trafficking in model cell systems. They are currently extending these findings to brain preparations. The ability to test hypotheses about DAT derived in model systems, back in the brain, remains one of the most important and unique aspects of Dr. Zahniser's work. Continued K05 support will provide her protected time to focus on new research directions and mentoring, as well as to utilize her recent leadership training in organizing a new university-wide drug addiction research center, integrating basic, clinical and treatment research. Dr. Zahniser contributed/s to NIDA's mission by serving on NIDA Council and IRP Board of Scientific Counselors, external advisory boards for other addiction centers, and as a reviewer. She is active in mentoring students, postdoctoral fellows and junior faculty who are supported by NIDA. Ten of her 20 former trainees are faculty members/senior fellows who continue independently to focus on drug/alcohol addiction research.